The invention is a fast adapting control system and method that is particularly useful for tracking in an active acoustic attenuation system.
Active acoustic attenuation involves injecting a canceling acoustic wave to destructively interfere with and cancel a system input acoustic wave to yield a system output acoustic wave. In an active acoustic attenuation system, an adaptive control filter receives a reference signal and in turn supplies a correction signal to an output transducer such as a loudspeaker in a sound application or a shaker in a vibration application. The output transducer injects the canceling acoustic wave or secondary input to destructively interfere with the system input so that the system output is zero or some other desired value.
The system output acoustic wave is sensed with an error sensor such as a microphone in a sound system, or an accelerometer in a vibration system. The error sensor generates an error signal in response to the system output. An error input signal, which depends at least in part on the error signal, is supplied to the adaptive control filter, and adaptive parameters in the control filter are updated in relation to the error input signal to adapt the filter. A convergence factor or step size parameter .mu. is normally selected to ensure convergence of the adaptive control filter.
It is important that the adaptive control filter in an active acoustic attenuation system be stable (i.e. converge), and also that the adaptive filter be robust. One consideration in this respect, is that the adaptive control filter account for propagation delay and phase shifts in an auxiliary path between the output of the adaptive control filter and the output of the error sensor. The filtered-X least-means-square (LMS) and the filtered-U recursive-least-means-square (RLMS) update methods as described in U.S. Pat. No. 4,677,676 which is incorporated herein by reference, account for the delay and phase shifts in the auxiliary path when updating the adaptive control filter model, and are effective means of providing adaptive control in many active acoustic attenuation systems. In the filtered-X and filtered-U methods, it is normally preferred that C modeling of the auxiliary path be accomplished adaptively on-line such as described in above incorporated U.S. Pat. No. 4,677,676. Other methods such as delayed inverse C modeling, or delayed Hermetian transpose C modeling can also be used to account for delay and phase shift in the auxiliary path.
Even if these methods are used, propagation delay in the auxiliary path can cause some instability in the adaptive control filter model if the convergence factor or step size .mu. is too large.